This Application claims priority from Korean Patent Application No. 2002-12222, the contents of which are incorporated herein by reference.
1. Field
This disclosure teaches techniques related to a transconductor. Specifically, techniques related to a transconductor capable of extending an input signal restriction range for securing stability of an element are taught.
2. Background of the Related Art
A transconductor is used for outputting a current signal corresponding to an input signal size. In performing this function, the transconductor forms a basic circuit element for circuits like a variable amplifier, a filter, etc.
A variety of such transconductors are known. Some of which are disclosed herein. A paper titled xe2x80x9cA 20-MHz sixth-order BiCMOS parasitic insensitive continuous-time filter and second-order equalizer optimized for disc-drive read channelsxe2x80x9d, IEEE J, Solid-State Circuits, vol.28, pp.462-470, April.1993, discloses a transconductor using a BiCMOS element.
A conventional circuit for a transconductor disclosed in the above paper, is illustrated in FIG. 1. In the transconductor illustrated in FIG. 1, a drain-source voltage of a MOS(Metal-Oxide Semiconductor) transistor M1 is operated together with a control voltage(Vc) so that a constant voltage may be maintained, thereby achieving transconductance.
In the transconductor circuit with the structure shown in FIG. 1, however, a current flowing through a base of a bipolar transistor Q2 changes depending on a size of an input signal Vi. A base-emitter voltage of the bipolar transistor Q2 is changed accordingly. This results in a drain-source voltage of the MOS transistor M1 also being changed. A disadvantage in such a situation is that the transconductance is not maintained a constant.
Another paper by WYSZYNSKI, A, titled xe2x80x9clow voltage CMOS and BiCMOS triode transconductors and integrators with gain-enhanced linearity and output impedancexe2x80x9d, Electron. Lett., 1994, 30, pp211-213, discloses a conventional transconductor circuit, which is illustrated in FIG. 2.
A transconductor circuit as shown in FIG. 2 employs a feedback structure using an OP amp.(Operational Amplifier). However, because of the OP amp. used for this transconductor circuit, a substantial amount of power is dissipated. Further, in such a structure, a drain voltage cannot be made lower than a threshold voltage due to common-mode range(CMR) restrictions. Therefore, a disadvantage in such a structure is that the variable range for the transconductor will be restricted. Further, another disadvantage is that the feedback structure is not adequate for high frequency use, at least because, a low frequency pole is generated.
Another paper by P. Likittanapong, A. Worapshet and C. Toumazou, titled xe2x80x9cLinear CMOS triode transconductor for low voltage applicationsxe2x80x9d, Electronics Letters, Jun. 11, 1998, vol. 34, No.12, discloses yet another conventional transconductor that also employs a feedback structure without using an OP amp. This is illustrated in FIG. 3.
However, the transconductor shown in FIG. 3 is disadvantageous at least because it is not adequate for a circuit intended for high frequency use. This again is at least because a low frequency pole is generated.
Still another conventional transconductor is disclosed in a paper by Fuji Yang, Christian C., titled xe2x80x9cA Low-Distortion BiCMOS Seventh-Order Bessel Filter Operationg at 2.5V Supplyxe2x80x9d, IEEE J. Solid-State Circuits, vol. 31, No. 3, March 1996. This transconductor employs a structure involving crossing pairs, which is illustrated in FIG. 4.
A transconductor shown in FIG. 4 is capable of maintaining a drain voltage of a MOS transistor M1 at a constant. Since this structure does not use a feedback, it can be used under high frequency conditions. A base-emitter voltage VBE4 of a bipolar transistor Q4, in FIG. 4, can be obtained by the following formula.
VBE4=Vrefxe2x88x92VBE1xe2x80x83xe2x80x83[Formula 1]
Here, VBE1 is a base-emitter voltage of a bipolar transistor Q1.
As can be seen from formula 1, a base-emitter voltage VBE4 of the bipolar transistor Q4, whose emitter terminal is connected to the ground, varies depending on a bias voltage applied to the Vref. Therefore, if a high voltage, for example, a voltage more than 1.5V is applied to the Vref, an excessive current flows to the bipolar transistor Q4. Because of such an excessive current an element could be destroyed. Therefore, a disadvantage of such a structure as shown in FIG. 4 is that it can be used only with low voltages.
As can be seen from the above discussions, a transconductor capable of operating even in high voltage condition, having a wide range of operation frequency while maintaining linearity is required.
The teachings of the present disclosure are aimed at overcoming some of the disadvantages noted in relation to conventional techniques.
To overcome the disadvanteages noted above there is provided a transconductor for generating a current corresponding to an input voltage. The transconductor has a crossing pairs structure. The transconductor comprises a first and a second MOS(Metal-Oxide Semiconductor) transistors mutually connected in series to a voltage source. A first bipolar transistor is connected to a current source. A collector terminal of the first bipolar terminal is connected to an output current terminal. An emitter terminal of the first bipolar terminal is connected to a gate terminal of the second MOS transistor. A second bipolar transistor is connected in series to the first bipolar transistor. A base terminal of the second bipolar transistor is connected to a node between the first MOS transistor and the second MOS transistor. A third MOS transistor is provided. A gate terminal of the third MOS transistor is connected to an input terminal for a signal from outside. A drain terminal of the third MOS transistor is connected to an emitter terminal of the second bipolar transistor.
In a specific improvement the transconductor further includes a reference bias voltage source provided to a base terminal of the first bipolar transistor; and a control voltage source applied to a base terminal of the first MOS transistor for adjusting transconductance.
More specifically, the control voltage source is preferably configured such that a voltage whose value is a sum of a voltage intended for being maintained between a gate and a source of the third MOS transistor and the reference bias voltage sources, is applied to a base terminal of the first MOS transistor.
Additional advantages, objects, and features of the invention will be set forth in part in the description which follows and in part will become apparent to those having ordinary skill in the art upon examination of the following or may be learned from practice of the invention. The objects and advantages of the invention may be realized and attained as particularly pointed out in the appended claims.